Women with hormone dependent breast cancer experience objective tumor regression in response to initial hormonal therapy but relapses uniformly take place. Second, third and fourth line hormonal therapies frequently cause additional remissions but the mechanisms responsible are poorly understood. As an explanation for these effects, tumors may adapt in response to hormonal therapy and develop "hypersensitivity" to estrogen through up-regulation of signaling pathways. This results in the development of enhanced responsiveness to the proliferative and pro-apoptotic effects of estradiol. This process of "Adaptive Hypersensitivity" has important implications for the therapy of women with breast cancer. A working hypothesis states that synergistic interactions among the c-Myc, MAP kinase and mTOR pathways, acting at the level of the cell cycle, result in hypersensitivity to the proliferative effects of estradiol. Type II cellular mechanisms involving death receptor pathways and mitochondria enhance sensitivity to the pro-apoptotic effects of estradiol. This proposal seeks to determine the precise mechanisms mediating hypersensitivity. A major priority is to translate these findings into methods to prevent the development of resistance to hormonal therapy and to enhance the pro-apoptotic effects of estradiol. The ultimate goal is to develop a three pronged treatment approach involving (a) maximum deprivation of estradiol (b) concomitant blockade of signaling pathways to abrogate "adaptive hypersensitivity" to the proliferative effects of estradiol and (c) cyclic administration of high dose estrogen with enhancers of apoptosis to kill adapted cells. This grant proposal addresses four aims: Specific Aim 1: Determine the mechanisms for hypersensitivity to the proliferative effects of estradiol. These studies will utilize induction and reversion strategies to examine the precise roles of the MAP kinase, mTOR and c-Myc pathways in the induction of hypersensitivity and will determine if TSC-1, TSC-2, and Rheb are involved in mTOR up-regulation. Specific Aim 2: Determine the mechanisms for hypersensitivity to the pro-apoptotic effects of estradiol and establish that Type II mechanisms take place, which involve death receptor and mitochondrial pathways. Specific Aim 3: Enhance estradiol induced apoptosis and utilize pharmacologic agents to prevent hypersensitivity to the proliferative effects of estradiol Three approaches are designed to enhance estradiol induced apoptosis including use of an anti-sense drug directed against Bcl-2, anti-Fas activating antibodies, and the DR3 ligand, TLIA. The mTOR inhibitors FTS, CCI-779, and rapamycin are employed to block growth factor pathways and abrogate hypersensitivity. Specific Aim 4: Develop treatment strategies to translate observations about estradiol induced apoptosis into practically applicable clinical therapies. The efficacy of cyclic therapy with sequential use of estradiol deprivation and high dose estrogen administration with enhancers of apoptosis is examined. A clinical trial in women, which is designed to specifically demonstrate that high dose estrogen induces tumor regression through the mechanism of apoptosis will be conducted.